In an electric car on which a linear induction motor of three phases is mounted, a VVVF inverter generates a three-phase AC voltage by DC power fed to the electric car, and the linear induction motor is driven by the three-phase AC voltage. This VVVF inverter is controlled by an inverter control unit, and it generates the three-phase AC voltage at a frequency obtained by adding or subtracting a slip frequency to or from the synchronizing frequency of the linear induction motor.
In general, the synchronizing frequency of the linear induction motor is calculated on the basis of the frequency of the output pulses of a pulse generator which is disposed on the predetermined axle of the electric car. However, when a wheel attached to the axle to have its rotational frequency detected, on which the pulse generator is disposed, has worn away, the wheel diameter of the wheel changes, a cycle per revolution of the wheel changes, and the frequency of the output pulses of the pulse generator changes, so that the synchronizing frequency needs to be corrected in accordance with the wheel diameter.
In general, the change of the wheel diameter attendant upon the wear of the wheel is coped with in such a way that wheel-diameter data means is disposed in the driver's cabin of the electric car, and that wheel diameter data measured in the routine inspection of the electric car is stored in the wheel-diameter data means. The wheel diameter data stored in the wheel-diameter data means is transmitted from the driver's cabin to the inverter control unit. In the inverter control unit, the synchronizing frequency is corrected using the transmitted wheel-diameter information. However, in a case where the wheel-diameter data means in the driver's cabin has gotten out of order, or where an error has developed in the transmission of the wheel diameter data to the inverter control unit, so that the wheel diameter data from the wheel-diameter data means cannot be normally received by the inverter control unit, the correction of the synchronizing frequency cannot be made.
JP61-231805A concerning a prior-art technique discloses an apparatus wherein the absolute velocity of an electric car is evaluated by employing beacons, and a synchronizing frequency is corrected on the basis of the absolute velocity of the electric car. The apparatus disclosed in the prior-art technique, however, needs to especially dispose the beacons, and an absolute velocity sensor corresponding to the beacons. The beacons need to be especially disposed in a plurality of places on the ground, and they require maintenance and inspection on the ground.
Patent Document 1: JP61-231805A